Acidithiobacillus ferrooxidans mediates morphology evolution of schwertmannite in the presence of Fe2+

Schwertmannite (Sch) possessing a typical "hedgehog" morphology is ubiquitously found in acid mine drainage (AMD) through biological oxidation of Fe2+ by Acidithiobacillus ferrooxidans (Acidithiobacillus ferrooxidans). However, little information is available on the effect of A. ferrooxidans on the morphology evolution of Sch during its formation. In this study, it was found that Fe2+ and A. ferrooxidans (with the presence of Fe2+) could promote the "hedgehog" structure formation of Sch in acid aerobic environment, respectively. Aerobic systems led to the increase in needle-like goethite on the surface of ellipsoidal aggregates with comparison to anaerobic systems. Hydroxyl radical (center dot OH) produced only in the aerobic systems facilitated the formation of Fe3+. Meanwhile, A. ferrooxidans, containing mainly tryptophan, could significantly change the interfacial potential and favor the electron transfer from Fe2+ to Sch. Moreover, it also could coordinate with Fe (Fe2+, Fe3+). Thus, a possible mechanism for developing a typical "hedgehog" morphology of biogenic Sch was that Fe3+ from center dot OH oxidization of Fe2+ and/or released from Sch would chelate with tryptophan to form Fe3+-tryptophan complex, and further induce the formation of needle-like goethite. However, during the crystal growth of goethite, tryptophan would be released to solution. These findings have important implications for comprehensively understanding the role of microorganism on mineral formation in AMD environment.